Species differences in grouping: estrildid finches

 

Species-typical patterns of grouping have profound impacts on many aspects of physiology and behavior. However, prior to our studies in estrildid finches, neural mechanisms that titrate species-typical group size preferences, independent of other aspects of social organization (e.g., mating system and parental care), have been wholly unexplored, likely because species-typical group size is typically confounded with other aspects of behavior and biology.

An additional complication is that components of social organization are evolutionarily labile and prone to repeated divergence and convergence. Hence, we cannot assume that convergence in social structure has been produced by convergent modifications to the same neural characters, and thus any comparative approach to grouping must include not only species that differ in their species-typical group sizes, but also species that exhibit convergent evolution in this aspect of social organization. 

Using five estrildid finch species that differ selectively in grouping (all biparental and monogamous) we have demonstrated that neural motivational systems evolve in predictable ways in relation to species-typical group sizes, including convergence in two highly gregarious species and convergence in two relatively asocial, territorial species. These systems include nonapeptide (vasotocin and mesotocin) circuits that encode the valence of social stimuli (positive-negative), titrate group-size preferences, and modulate anxiety-like behaviors.

Nonapeptide systems exhibit functional and anatomical properties that are biased towards gregarious species, and experimental reductions of nonapeptide signaling by receptor antagonism and antisense oligonucleotides significantly decrease preferred group sizes in the gregarious zebra finch. Combined, these findings suggest that selection on species-typical group size may reliably target the same neural motivation systems when a given social structure evolves independently.

Seasonal variation in territoriality and flocking: emberizid sparrows

 

The finch species described above exhibit year-round, evolved differences in grouping behavior, and thus it remains to be determined whether seasonal or conditional variation in grouping behavior relies upon similar neural mechanisms.

To address this question, we have conducted a variety of recent studies in North American sparrow species that are territorial during the breeding season, but vary in their nonbreeding social structures. These includes species that are territorial, flocking, or loosely dispersed in the fall and winter.

Anatomical studies demonstrate that flocking in sparrows is associated with some of the same nonapeptide mechanisms that we have identified as being important for flocking in finches (see above), although there are important differences, as well. Pharmacological experiments are currently underway in sparrows in order to directly demonstrate the importance of nonapeptide signaling for winter flocking.

In addition to the nonapeptides, these studies have focused on several other neuropeptides and enzymes, all of which appear to be relevant to species differences in social structure. One of these is vasoactive intestinal polypeptide (VIP), a neuromodulator that is currently a focus of numerous studies, as described in the next section.

 

Behavioral biology of vasoactive intestinal polypeptide (VIP)

 

VIP cells, fibers and receptors are found in virtually every brain region that is important for social behavior, but very few studies have examined the behavioral functions of this peptide. There are numerous VIP cell groups in the brain and likely a diversity of functional circuits, and multiple lines of evidence suggest that these circuits may be variably important for grouping behavior, aggression, and parental care.

All of these topics are currently under investigation under support from the National Institutes of Health. In estrildid finches, we find that flocking species have higher densities of VIP binding sites (receptors) in select brain loci than do territorial species, and those loci also show a higher density of VIP innervation during winter in sparrows that flock than those that do not.

Consistent with these observations, pharmacological manipulations in zebra finches demonstrate that endogenous VIP is an important modulator of social contact, group-size preference, and response to social novelty. An additional set of data demonstrates that VIP neurons of the anterior hypothalamus potently and selectively promote aggression in both territorial and gregarious finches. In addition, VIP innervation of the anterior hypothalamus in sparrows correlates positively with individual and species differences in aggression, suggesting that hypothalamic VIP is a target of selection during behavioral evolution.

 

For more information about our current work, see our Finch and Sparrow pages.